1. Field of the Invention
The present invention relates to driving a liquid crystal display and, more particularly, to a liquid crystal display (LCD) driving circuit in which a small number of voltage sources enable multi-gradation display, and gamma correction can be obtained without extra gamma correction circuits.
2. Discussion of the Related Art
A liquid crystal display (LCD) has generally been mounted in a personal computer (PC). Currently, a standard LCD displays 512 colors using a 6 bit digital driver.
Since image information and communication information have been available in a computer, an LCD has been required to display more than 64 gradations (0.26 million colors) for each of R, G, and B. Currently, the transmittivity (luminosity of an amount of transmitted light) of an LCD is changed according to an applied voltage, as shown in the T-V (transmission-voltage) curve of FIG. 1. Displaying methods are divided into a normally white and a normally black, depending on whether the state in which a voltage is not applied results in a white mark or a black one. FIG. 1 applies to the case of the normally black mode. Referring to FIG. 1, the change of transmittivity is nonlinear according to the change in voltage in sections o-a and c-d, and more linear in section a-c. The goal of gamma correction is to make the whole T-V curve linear by making sections o-a and c-d linear. As shown in FIG. 2, line 3, which represents the change in voltage of the T-V curve, is corrected with line 1, thus making the T-V line linear, as shown by line 2.
FIG. 3 is a block diagram of a conventional gamma correction circuit, which includes an analog/digital (A/D) converter 11 for converting an analog image signal into a digital signal; a detecting part 12 for determining a digital value converted by the A/D converter 11 and providing an address for a lookup table 13 having a gamma value; a lookup table 13 for outputting the gamma value stored in the address provided by the detecting part 12; an adder 14 for adding an output of the A/D converter 11 and an output of the lookup table 13; and a D/A converter 15 for converting an output coming from the adder 14 into an analog signal.
The analog image signal inputted is converted into a digital value by the A/D converter 11 and finally is outputted into the detecting part 12 and the adder 14. The detecting part 12 determines the digital value converted and outputs a corresponding address into the lookup table 13. The lookup table 13 outputs into the adder 14 a gamma value recorded at the address provided by the detecting part 12. The gamma value necessary to make the T-V curve linear is already recorded in the lookup table 13. Accordingly, the output of the A/D converter 11 and that of the lookup table 13 are added in the adder 14, and then the added value is converted into an analog signal in the D/A converter 15, thereby making the T-V curve linear, like the line 2 of FIG. 2.
However, preciseness of the gamma correction is cut off due to a round off error which is generated when the analog signal is converted into the digital signal. Also, as a voltage source is increased, gradations are difficult to display, requiring an extra circuit and memory to perform gamma correction. As a result, the entire circuit is enlarged, increasing power consumption and manufacturing cost.
Referring to FIG. 4, there is another method for making a T-V curve linear by carrying out gamma correction, namely, by using ramp waves. In FIG. 4, "t1" is a time which is spent to increase one count value. Accordingly, if the tilt of a ramp is constant, there will be a voltage V2 between a voltage V1 at a time point ta and a voltage V3 at a time point tb for the gamma correction, and the counter should be fast to obtain V2.
For example, in FIG. 4, it is sufficient that a counter can count up to 64 in the case of 6 bit data. However, since a time point between ta and tb is needed to obtain the V2, about 8 bits should be processed during the same time. Accordingly, a lookup table shown in FIG. 3, such as a read only memory (ROM), is needed and a high speed counter is needed to process the extended data. This enlarges the entire circuit, increasing power consumption and manufacturing cost.